# The `app` attribute This is the smallest possible RTIC application: ``` rust {{#include ../../../../examples/smallest.rs}} ``` All RTIC applications use the [`app`] attribute (`#[app(..)]`). This attribute must be applied to a `mod`-item. The `app` attribute has a mandatory `device` argument that takes a *path* as a value. This must be a full path pointing to a *peripheral access crate* (PAC) generated using [`svd2rust`] **v0.14.x** or newer. More details can be found in the [Starting a new project](./new.md) section. The `app` attribute will expand into a suitable entry point so it's not required to use the [`cortex_m_rt::entry`] attribute. [`app`]: ../../../api/cortex_m_rtic_macros/attr.app.html [`svd2rust`]: https://crates.io/crates/svd2rust [`cortex_m_rt::entry`]: ../../../api/cortex_m_rt_macros/attr.entry.html ## `init` Within the `app` module the attribute expects to find an initialization function marked with the `init` attribute. This function must have signature `fn(init::Context) [-> init::LateResources]` (the return type is not always required). This initialization function will be the first part of the application to run. The `init` function will run *with interrupts disabled* and has exclusive access to Cortex-M where the `bare_metal::CriticalSection` token is available as `cs`. And optionally, device specific peripherals through the `core` and `device` fields of `init::Context`. `static mut` variables declared at the beginning of `init` will be transformed into `&'static mut` references that are safe to access. [`rtic::Peripherals`]: ../../api/rtic/struct.Peripherals.html The example below shows the types of the `core`, `device` and `cs` fields, and showcases safe access to a `static mut` variable. The `device` field is only available when the `peripherals` argument is set to `true` (it defaults to `false`). ``` rust {{#include ../../../../examples/init.rs}} ``` Running the example will print `init` to the console and then exit the QEMU process. ``` console $ cargo run --example init {{#include ../../../../ci/expected/init.run}} ``` ## `idle` A function marked with the `idle` attribute can optionally appear in the module. This function is used as the special *idle task* and must have signature `fn(idle::Context) - > !`. When present, the runtime will execute the `idle` task after `init`. Unlike `init`, `idle` will run *with interrupts enabled* and it's not allowed to return so it must run forever. When no `idle` function is declared, the runtime sets the [SLEEPONEXIT] bit and then sends the microcontroller to sleep after running `init`. [SLEEPONEXIT]: https://developer.arm.com/docs/100737/0100/power-management/sleep-mode/sleep-on-exit-bit Like in `init`, `static mut` variables will be transformed into `&'static mut` references that are safe to access. The example below shows that `idle` runs after `init`. **Note:** The `loop {}` in idle cannot be empty as this will crash the microcontroller due to a bug in LLVM which miss-optimizes empty loops to a `UDF` instruction in release mode. ``` rust {{#include ../../../../examples/idle.rs}} ``` ``` console $ cargo run --example idle {{#include ../../../../ci/expected/idle.run}} ``` ## Hardware tasks To declare interrupt handlers the framework provides a `#[task]` attribute that can be attached to functions. This attribute takes a `binds` argument whose value is the name of the interrupt to which the handler will be bound to; the function adorned with this attribute becomes the interrupt handler. Within the framework these type of tasks are referred to as *hardware* tasks, because they start executing in reaction to a hardware event. The example below demonstrates the use of the `#[task]` attribute to declare an interrupt handler. Like in the case of `#[init]` and `#[idle]` local `static mut` variables are safe to use within a hardware task. ``` rust {{#include ../../../../examples/hardware.rs}} ``` ``` console $ cargo run --example hardware {{#include ../../../../ci/expected/hardware.run}} ``` So far all the RTIC applications we have seen look no different than the applications one can write using only the `cortex-m-rt` crate. From this point we start introducing features unique to RTIC. ## Priorities The static priority of each handler can be declared in the `task` attribute using the `priority` argument. Tasks can have priorities in the range `1..=(1 << NVIC_PRIO_BITS)` where `NVIC_PRIO_BITS` is a constant defined in the `device` crate. When the `priority` argument is omitted, the priority is assumed to be `1`. The `idle` task has a non-configurable static priority of `0`, the lowest priority. When several tasks are ready to be executed the one with *highest* static priority will be executed first. Task prioritization can be observed in the following scenario: an interrupt signal arrives during the execution of a low priority task; the signal puts the higher priority task in the pending state. The difference in priority results in the higher priority task preempting the lower priority one: the execution of the lower priority task is suspended and the higher priority task is executed to completion. Once the higher priority task has terminated the lower priority task is resumed. The following example showcases the priority based scheduling of tasks. ``` rust {{#include ../../../../examples/preempt.rs}} ``` ``` console $ cargo run --example preempt {{#include ../../../../ci/expected/preempt.run}} ``` Note that the task `gpiob` does *not* preempt task `gpioc` because its priority is the *same* as `gpioc`'s. However, once `gpioc` terminates the execution of task, `gpiob` is prioritized over `gpioa` due to its higher priority. `gpioa` is resumed only after `gpiob` terminates. One more note about priorities: choosing a priority higher than what the device supports (that is `1 << NVIC_PRIO_BITS`) will result in a compile error. Due to limitations in the language, the error message is currently far from helpful: it will say something along the lines of "evaluation of constant value failed" and the span of the error will *not* point out to the problematic interrupt value -- we are sorry about this!